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Versions: (draft-baker-homenet-prefix-assignment) 00 01 02 03 04 05 06 07 08 RFC 7695

Network Working Group                                         P. Pfister
Internet-Draft                                               B. Paterson
Intended status: Standards Track                           Cisco Systems
Expires: April 27, 2015                                         J. Arkko
                                                                Ericsson
                                                        October 24, 2014


            Prefix and Address Assignment in a Home Network
                draft-ietf-homenet-prefix-assignment-01

Abstract

   This memo describes a home network prefix and address assignment
   algorithm running on top of any 'flooding protocol' that fulfills the
   specified requirements.  It is expected that home border routers are
   allocated one or multiple IPv6 prefixes through DHCPv6 Prefix
   Delegation (PD) or that prefixes are made available through other
   means.  An IPv4 address can also be assigned and private addresses be
   used with NAT to provide IPv4 connectivity.  In both cases, provided
   prefixes need to be efficiently divided among the multiple links, and
   routers need to obtain addresses.  This document describes a
   distributed algorithm for IPv4 and IPv6 prefixes division, assignment
   and router's address assignment, and specifies how hosts can be given
   addresses and configuration options using DHCP or SLAAC.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 27, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .   4
   3.  Prefix and Address Assignment Algorithms' Outline . . . . . .   4
   4.  Router Behavior . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Data structures . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Routers' Interfaces . . . . . . . . . . . . . . . . . . .   7
     4.3.  Obtaining a Delegated Prefix  . . . . . . . . . . . . . .   7
     4.4.  Network Leader  . . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Designated Router . . . . . . . . . . . . . . . . . . . .   8
       4.5.1.  Sending Router Advertisement  . . . . . . . . . . . .   9
       4.5.2.  DHCP Server Operations  . . . . . . . . . . . . . . .   9
     4.6.  Applying an Assignment on an Interface  . . . . . . . . .  10
     4.7.  DNS Support . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Flooding Protocol Requirements  . . . . . . . . . . . . . . .  11
     5.1.  Router ID . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Propagation Delay . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Flooding Assigned Prefixes  . . . . . . . . . . . . . . .  12
     5.4.  Flooding Delegated Prefixes . . . . . . . . . . . . . . .  12
     5.5.  Flooding Routers' Address Assignments . . . . . . . . . .  12
   6.  Prefix Assignment Algorithm . . . . . . . . . . . . . . . . .  13
     6.1.  When to execute the Prefix Assignment Algorithm . . . . .  13
     6.2.  Assignment Precedence . . . . . . . . . . . . . . . . . .  14
     6.3.  Testing Assignment's validity . . . . . . . . . . . . . .  14
     6.4.  Testing Assignment's availability . . . . . . . . . . . .  14
     6.5.  Accepting an Assigned Prefix  . . . . . . . . . . . . . .  14
     6.6.  Making a New Assignment . . . . . . . . . . . . . . . . .  15
     6.7.  Using Authoritative Prefix Assignments  . . . . . . . . .  16
     6.8.  Choosing the Assignment's Priority  . . . . . . . . . . .  17
     6.9.  Prefix Assignment Algorithm steps . . . . . . . . . . . .  17
     6.10. Downstream DHCPv6 Prefix Delegation support . . . . . . .  18
   7.  Address Assignment Algorithm  . . . . . . . . . . . . . . . .  19
     7.1.  Router's address pools  . . . . . . . . . . . . . . . . .  20
     7.2.  Address Assignment Algorithm  . . . . . . . . . . . . . .  20
   8.  Hysteresis Principle  . . . . . . . . . . . . . . . . . . . .  21
     8.1.  Prefix and Address assignments  . . . . . . . . . . . . .  21
     8.2.  Delegated Prefixes  . . . . . . . . . . . . . . . . . . .  21



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       8.2.1.  Unreliable uplink . . . . . . . . . . . . . . . . . .  21
       8.2.2.  Unreliable in-home link . . . . . . . . . . . . . . .  22
   9.  ULA and IPv4 Prefixes Generation  . . . . . . . . . . . . . .  22
     9.1.  ULA Prefix Generation . . . . . . . . . . . . . . . . . .  22
       9.1.1.  Choosing the ULA prefix . . . . . . . . . . . . . . .  23
       9.1.2.  Advertising a ULA prefix  . . . . . . . . . . . . . .  23
       9.1.3.  Extending prefix lifetime . . . . . . . . . . . . . .  24
       9.1.4.  Authoritative ULAs  . . . . . . . . . . . . . . . . .  24
     9.2.  IPv4 Private Prefix Generation  . . . . . . . . . . . . .  24
   10. Manageability Considerations  . . . . . . . . . . . . . . . .  24
   11. Documents Constants . . . . . . . . . . . . . . . . . . . . .  25
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     13.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Scarcity Avoidance Mechanism . . . . . . . . . . . .  28
     A.1.  Prefix Wasts Avoidance  . . . . . . . . . . . . . . . . .  29
     A.2.  Increasing Assigned Prefix Length . . . . . . . . . . . .  30
     A.3.  Foreseeing Prefixes Exaustion . . . . . . . . . . . . . .  30
     A.4.  Cutting an Existing Assignment  . . . . . . . . . . . . .  31
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   This memo describes a fully distributed prefix and address assignment
   algorithm for home networks, running on top of any 'flooding
   protocol' that fulfills the specified requirements.  It is expected
   that home border routers are allocated one or multiple IPv6 prefixes
   through DHCPv6 Prefix Delegation (PD) [RFC3633] or that prefixes are
   made available through other means.  When an IPv4 address is
   assigned, a home private IPv4 prefix may be used with NAT to provide
   IPv4 connectivity to the whole home, as well as Unique Local Address
   prefixes [RFC4193] may be used in order to provide internal
   connectivity whenever global IPv6 connectivity is not available.

   Obtained IPv6 or IPv4 prefixes need to be efficiently divided among
   the multiple links.  For the purposes of this document, we refer to
   this process as prefix assignment.  This memo describes an algorithm
   for such prefix division, assignment and router's address assignment,
   as well as the way hosts can be given addresses and configuration
   options using DHCPv4 [RFC2131], DHCPv6 [RFC3315] or SLAAC [RFC4862].
   In the rest of this document DHCP refers to both DHCPv4 and DHCPv6.

   Although this document recommends the use of 64 bits long prefixes,
   the algorithm do not require routers to assign prefixes of particular
   lengths.  When a delegated prefix is too small considered the number
   of links in the home network, higher priority links may be privileged



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   or smaller prefixes can be assigned in order to avoid prefix
   scarcity.

   The rest of this memo is organized as follows.  Section 2 defines the
   usual keywords, Section 3 outlines the algorithms functioning and
   features, Section 4 describes how a home router behaves when running
   the prefix and address assignment algorithm.  Requirements for the
   underlying flooding protocol are detailed in Section 5.  The prefix
   assignment algorithm is detailed in Section 6 and Section 7 focuses
   on the address assignment algorithm.  Section 8 explains the
   hysteresis principles applied to both prefix and address assignments,
   Section 9 specifies the procedures for automatic generation of ULA
   and IPv4 prefixes, Section 10 explains what administrative interfaces
   are useful for advanced users that wish to manually interact with the
   mechanisms, Section 11 gives values for the constants used in this
   document, Section 12 discusses the security aspects and finally,
   Appendix A provides implementation guidelines for the optional
   scarcity avoidance mechanism.

   The Prefix Assignment Algorithm was first detailed in
   [I-D.arkko-homenet-prefix-assignment].  This document is a
   continuation and generalization of that draft to any underlying
   flooding protocol.  It also adds support for arbitrary prefix length,
   IPv4, scarcity avoidance mechanism or manual configuration.

2.  Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
   "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [RFC2119].

3.  Prefix and Address Assignment Algorithms' Outline

   Given one or multiple prefixes for the entire network, each prefix is
   subdivided by the prefix assignment algorithm so that every link is
   given one assignment per available prefix.  Assignments are
   advertised through the whole network using the underlying flooding
   protocol, collisions are detected and valid assignments are chosen
   and applied on every link.  Once a prefix is applied, hosts and
   routers may be given addresses.  In summary, the algorithm works in
   four steps:

   1.  The home is given IPv6 or IPv4 prefixes called Delegated Prefixes
       (DPs).

   2.  Each link is provided an Assigned Prefix (AP) from each available
       Delegated Prefix.




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   3.  Routers internally check for AP's validity and select Chosen
       Prefixes (CPs).

   4.  Once a link is given an assignment, routers may get addresses
       from specified address pools and hosts may be configured using
       SLAAC or by the per-link elected DHCP server.

   This algorithm, which intends to fulfill requirements specified in
   [I-D.ietf-homenet-arch], has the following features:

   o  Each delegated prefix is effectively divided so that each link is
      assigned a reasonable part.  If the delegated prefix is too small
      given the size of the network, prefixes of arbitrary lengths may
      be used.

   o  The algorithm is completely distributed.  Routers may join or
      leave and DPs may be added or removed at any time.

   o  IPv4 connectivity is provided when a home router acquires an IPv4
      address and default route from an external source.  In this case a
      private IPv4 delegated prefix is generated and prefixes are
      assigned similarly to IPv6.

   o  The network may spontaneously generate and use a Unique Local
      Address (ULA) prefix.

   o  Assignments are stable across reboots and some network changes
      (e.g. adding or removing routers).

   o  DHCP options like DNS servers, prefix colors
      [I-D.bhandari-dhc-class-based-prefix], or any upcoming options may
      be attached to each prefix and may be relayed down to the host
      when it is given addresses.

   o  The user can manually assign prefixes to links.  Such assignments
      will take precedence over automatically assigned prefixes.

   o  Assignments and interfaces can be given priorities.  When a
      delegated prefix is too small, such values may be used to
      prioritize prefix assignment to certain links.

4.  Router Behavior

   All home routers participating in the prefix assignment algorithm
   MUST fulfill the requirements defined in this document and use a
   common flooding protocol and routing protocol.  Classic CPE routers
   [RFC7084] are supported as downstream routers and dowstream DHCPv6-PD
   enabled routers are supported as both downstream and uplink routers,



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   but problems may occur when such router is connected to the home
   network on both WAN and LAN side.  In the later case, finer external
   interface detection algorithm or static configuration can be used to
   solve the issue, but these are out of the scope of this document.

4.1.  Data structures

   Each router MUST maintain a list of all the Delegated Prefixes.
   These prefixes may be locally generated, as described in Section 4.3,
   or come from other routers as described in Section 5.4.

   Each router MUST maintain a list of all the Assigned Prefixes
   advertised by other routers.  Each AP is learnt through the
   mechanisms described in Section 5.3 and is defined as a tuple of:

   Prefix:  The assigned prefix.

   Router ID:  The identifier of the advertising router.

   Link ID:  If the assignment is made on a connected link, an interface
         identifier of the interface connected to that link.

   Authoritative bit:  A boolean that tells whether the assignment comes
         from a network authority (DHCPv6 PD, manual configuration,
         etc...).

   Assignment's Priority:  A value between PRIORITY_MIN and
         PRIORITY_MAX, specifying the assignment's priority.

   The AP list is the result of the information provided by the flooding
   protocol, as specified in Section 5.3.

   The router MUST maintain a list of all prefixes currently chosen to
   be applied on connected links.  They are Chosen Prefixes (CPs) and
   described by a tuple of:

   Prefix:  The assigned prefix.

   Link ID:  An interface identifier of the interface connected to the
         link on which the assignment is made.

   Authoritative bit:  A boolean that tells whether the assignment comes
         from a network authority (DHCPv6 PD, manual configuration,
         etc...).

   Assignment's Priority:  A value between PRIORITY_MIN and
         PRIORITY_MAX, specifying the assignment's priority.




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   Advertised:  Whether that assignment is being advertised by the
         flooding protocol (see Section 5.3).

   Applied:  Whether that assignment is applied on link's configuration
         (see Section 4.6).

   Chosen Prefixes that are marked as 'Advertised' are distributed to
   other routers using the flooding protocol and are therefore
   considered as Assigned Prefixes by other routers.  The goal of the
   Prefix Assignment Algorithm is to ensure that all routers have a
   consistent view of Assigned Prefixes on each link.

   The router MUST maintain a database of its own address assignments,
   and address assignments made by other routers on connected links as
   learnt through means described in Section 5.5.

4.2.  Routers' Interfaces

   Each interface MUST either be considered as internal or external.
   Prefixes and addresses are only assigned to internal interfaces.  The
   criteria to make this distinction are out of the scope of this
   document.

   If an internal interface becomes external, all prefixes and addresses
   assigned on the considered interface MUST be deleted and no longer
   announced, and the prefix assignment algorithm MUST be run.

   If an external interface becomes internal, the prefix assignment
   algorithm MUST be run (see Section 6.1).

   Whenever two or more interfaces are connected to the same link, all
   but one of them SHOULD be ignored by the prefix assignment algorithm.
   A mechanism to detect such situation SHOULD be provided by the
   flooding algorithm.

4.3.  Obtaining a Delegated Prefix

   Delegated Prefixes (Of any kind: Global, ULA, IPv4...) can be
   obtained or generated through different means:

   o  It can be delegated by a service provider (DHCPv6 PD, 6rd
      [RFC5969], etc..).

   o  It can be provisioned by an administrative authority (user
      configuration, netconf [RFC6241], etc... ).

   o  A ULA prefix may be spontaneously generated as defined in
      Section 9.1.



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   o  An IPv4 private prefix may be spontaneously generated as defined
      in Section 9.2.

   DHCP options MAY be attached to a delegated prefix by the router that
   either generated the prefix or received it through DHCPv6 PD.  IPv6
   delegated prefix options MUST be encoded as DHCPv6 options.  IPv4
   delegated prefix options MUST be encoded as DHCPv4 options.

   As DHCP options are numerous and new ones may be defined, specifying
   routers' behavior regarding each option is out of the scope of this
   document.  In order to avoid misconfiguration, routers must follow
   the two following general rules:

   o  A router MUST NOT advertise a prefix obtained through DHCPv6 PD if
      it doesn't understand the all of the provided options.

   o  A router MUST NOT make or accept any assignment associated to a
      delegated prefix if it doesn't understand the all of the DHCP
      options advertised with the delegated prefix.

   The mif working group may provide useful inputs concerning the way
   the home network should handle different prefixes associated with
   heterogeneous uplinks.

4.4.  Network Leader

   A router considers itself as the Network Leader if and only if its
   router ID is greater than all other router IDs in received Prefix
   Assignments and Delegated Prefixes.

4.5.  Designated Router

   On a link where custom host configuration must be provided, or
   whenever SLAAC cannot be used, a DHCP server must be elected.  That
   router is called designated router and is dynamically chosen by the
   prefix assignment algorithm.

   A router MUST consider itself designated router on a given link if
   either one of the following conditions holds:

   o  The link's Assigned Prefixes list is empty. i.e. no other router
      is advertising assignments on the considered link.  And, if such
      information is provided by the flooding protocol, the router has
      the highest id on the link.

   o  Considering all APs and advertised CPs on the given link, the
      router is advertising the one with:




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      1.  The lowest authoritative bit.

      2.  In case of tie, the lowest priority.

      3.  In case of tie, the highest router ID.

         Note: That particular order (inverted compared to assignments'
         priority) is motivated by the few cases where a router may
         override an existing assignment by advertising an assignment of
         higher priority.  In such a case, the designated router should
         remain the same.

         Example: A new router is powered on and connected to another
         router that was already there (doing DHCP).  It sees the
         assigned prefix for their common link, but also has, in its own
         configuration, an authoritative assignment for the link.  It
         starts advertising the authoritative assignment, which causes
         the second router to remove its previous assignment.  Thanks to
         the inverted order, the DHCP server will remain the same.

4.5.1.  Sending Router Advertisement

   On a given link, the designated router MUST send router
   advertisements (RAs) including Prefix Information Options for all the
   Chosen Prefixes associated to that link.  SLAAC SHOULD be enabled
   when possible, unless the configuration states otherwise.  Prefixes
   valid and preferred lifetimes MUST be set to values lower or equal to
   the associated Delegated Prefix's valid and preferred lifetimes.

   Whenever an IPv6 default route is present in the RIB, the designated
   router MUST advertise itself as default router by specifying a
   strictly positive valid lifetime.  Whenever the last default route is
   removed, the designated router MUST send an RA with the valid and
   preferred lifetimes set to zero.

   The designated router MUST advertise itself as a router for all IPv6
   delegated prefixes using Route Information Options [RFC4191],
   independently of whether there is a default route or not.

4.5.2.  DHCP Server Operations

   On a given link, whenever SLAAC can't be used for all assignments, or
   DHCP configuration options must be provided to hosts, the designated
   router MUST act as a DHCP server and serve addresses on the given
   link.  A router MUST stop behaving as a DHCP server whenever it is
   not the link's designated router anymore.





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   Routers's addresses pool, specified in Section 7, MUST be excluded
   from DHCP hosts pools.

   The valid and preferred lifetimes MUST be set to values lower or
   equal to the associated Delegated Prefix's valid and preferred
   lifetimes.

4.6.  Applying an Assignment on an Interface

   Once a Chosen Prefix is created, a router first waits some time in
   order to detect possible collisions (Section 8).  Afterwards and if
   no collision is detected, the prefix is applied as follows:

   o  The router updates its interface configuration so that the prefix
      is assigned to the considered link.

   o  The router updates the routing protocol configuration so that it
      starts advertising the prefix.  Depending on the implementation,
      this step may not be needed as the routing protocol directly gets
      its configuration information from the interfaces configuration.

   o  If necessary, the router starts selecting an address for itself as
      defined in Section 7.

   o  If the router is the designated router on the considered link, it
      starts sending the Prefix Information Option with the considered
      prefix, as specified in Section 4.5.1.

   o  If the router is the designated router on the considered link and
      if the prefix requires DHCP configuration, it starts behaving as a
      DHCP server, as defined in Section 4.5.2, for the considered
      assigned prefix.

   When a prefix assignment is removed, the previous steps MUST be
   undone.  The router MUST also deprecate the prefix, if it had been
   advertised in Router Advertisements on an interface.  The prefix is
   deprecated by sending Router Advertisements with the PIO's preferred
   lifetime set to 0 [RFC4861].  Hosts that support DHCP reconfigure
   extension ([RFC3203], [RFC3315]) and that have been given leases MUST
   be reconfigured as well.

4.7.  DNS Support

   DHCP options attached to each delegated prefixes and propagated
   through the flooding protocol SHOULD contain the DHCP DNS options
   provided by the ISP (when provided).





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   Whenever the router knows which DNS server to use, or is acting as a
   DNS relay, it SHOULD include DNS DHCP options ([RFC3646]) within
   host's configuration messages and include the Router Advertisement
   DNS options ([RFC6106]) when sending RAs.

   DNS server selection in multi-homed networks is a complex issue that
   this document doesn't intend to solve.  One should look at IETF's mif
   working-group documents in order to obtain guidelines concerning DNS
   server selection.  It is RECOMMENDED that designated routers turns on
   a local DNS relay that fetches information from provided DNS servers.

5.  Flooding Protocol Requirements

   In this document, the Flooding Protocol (FP) refers to a protocol
   enabling information propagation to the whole network.  It was not
   specified in order to allow the working group to independently decide
   which routing protocol, configuration protocol, and prefix assignment
   method to use within the home network.  Routing protocol, like OSPFv3
   [RFC5340] (With its autoconf extension
   [I-D.ietf-ospf-ospfv3-autoconfig]) or IS-IS [RFC5308], could be
   extended in order to fulfill the requirements.  An independent
   protocol, for instance HNCP [I-D.ietf-homenet-hncp], could be used as
   well.

   The specified algorithm can use any protocol that fulfills the
   requirements specified in this section.

5.1.  Router ID

   The FP MUST provide a router ID.  ID collisions within the network
   MUST be rare and any conflicts MUST be resolved by the flooding
   protocol.  When the router ID is changed, the FP MUST immediately
   provide the new ID to the Prefix Assignment Algorithm, which will in
   turn be run again, without requiring the current state to be flushed.

   In the absence of collisions, the router ID MUST NOT be changed, and
   it SHOULD be stable across reboots, power cycling and router software
   updates.

5.2.  Propagation Delay

   The FP MUST provide an approximate upper bound of the time it takes
   for an update to be propagated to the whole network.  This value is
   referred to as the FLOODING_DELAY.  The algorithm ensures that, as
   long as the upper bound is respected, two identical prefixes will
   never be applied to different links, and two different prefixes will
   never be applied to the same link.  The algorithm and the network
   will recover when the upper bound is exceeded, but collisions may



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   appear in the routing protocol and errors may be propagated to upper
   layers.

   If the FP supports link-local flooding, which is used for router's
   address assignments, it SHOULD provide an approximate upper bound of
   the time it takes for an update to be propagated to a single link.
   This value is referred to as the FLOODING_DELAY_LL.  If link-local
   flooding is not available, or the value is not provided, the
   assignment algorithm MUST use the FLOODING_DELAY value instead.

5.3.  Flooding Assigned Prefixes

   The FP MUST provide a way to flood Chosen Prefixes marked as
   advertised and retrieve prefixes assigned by other routers (APs).
   Retrieved APs MUST contain all the information specified in
   Section 4.1.

5.4.  Flooding Delegated Prefixes

   The FP must provide a way to flood Delegated Prefixes and retrieve
   prefixes delegated to other routers.  Retrieved entries must contain
   the following information.

   Prefix:  The delegated prefix.

   Router ID:  The router ID of the router that is advertising the
         delegated prefix.

   Valid until:  A time value, in absolute local time, specifying the
         prefix validity time.

   Preferred until:  A time value, in absolute local time, specifying
         the prefix preferred time.

   DHCP information:  DHCP options attached to the delegated prefix.

   The FP MUST make sure time values are consistent throughout the
   network (i.e. differences are small compared to Delegated Prefixes
   lifetimes).  If no time synchronization protocol is used, the FP MUST
   keep track of prefix age across the network and within its database.

5.5.  Flooding Routers' Address Assignments

   Routers addresses are dynamically allocated, picked from a defined
   pool, and collisions must be detected using the FP.  The FP MUST
   provide a way to flood routers' addresses.  The flooding scope of
   those values SHOULD be link-local, but as addresses are unique within
   the home network, this is not mandatory.  For each address



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   assignment, the FP SHOULD provide the identifier of the interface
   connected to the link the address assignment was advertised on.

6.  Prefix Assignment Algorithm

   The Prefix Assignment Algorithm is a distributed algorithm that
   assigns one prefix from each available Delegated Prefix on every link
   that is considered to be internal by at least one connected router.
   The algorithm itself does not distinguish between global IPv6, ULA or
   IPv4 prefixes.  IPv4 prefixes are encoded as their IPv4-mapped IPv6
   form, as defined in [RFC4291] (i.e. ::ffff:A.B.C.D/X with X >= 96).

   When the Prefix Assignment Algorithm is executed, combinations of
   Delegated Prefixes and internal interfaces are being considered.  For
   the purpose of this discussion, the Delegated Prefix will be referred
   to as the current Delegated Prefix, and the interface will be
   referred to as the current Interface.  If a delegated prefix is
   included inside another delegated prefix, it is ignored.  This rule
   intends to ignore prefixes delegated from non-Homenet routers that
   previously obtained their larger prefix from one of Homenet's
   routers.

   The algorithm is specified here for the sake of clarity.  It can be
   optimized in some cases.  For instance Prefix Assignment deletion
   might not need to trigger algorithm's execution if all internal
   interfaces already have assignements associated to the same Delegated
   Prefix.  Similarly, when an ignored Delegated Prefix is deleted, it
   is not necessary to run the algorithm.  An implementation may work
   differently than specified here as long as the resulting behavior is
   identical to the behavior a router implementing this exact algorithm
   would have.

6.1.  When to execute the Prefix Assignment Algorithm

   The algorithm MUST be run whenever one of the following event occurs:

   o  A Delegated Prefix is created or deleted (A DP must be deleted
      when its lifetime is exceeded).

   o  A Prefix Assignment is created, deleted or modified.

   o  The router ID is modified.

   o  An external link becomes internal, or an internal link becomes
      external.






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   It is not required that the algorithm is synchronously run each time
   such an event occurs.  But the delay between the event and the
   algorithm execution MUST be small compared to FLOODING_DELAY.

6.2.  Assignment Precedence

   An assignment is said to take precedence over another assignment
   when:

   o  The authoritative bit value is higher.

   o  In case of tie, the priority value is higher.

   o  In case of tie, the advertising router's ID is higher.

6.3.  Testing Assignment's validity

   An Assigned Prefix or a Chosen Prefix is said to be valid if all the
   following conditions are met:

   1.  Its prefix is included in an advertised Delegated Prefix.

   2.  The prefix is not included or does not include any other Assigned
       Prefix with a higher precedence.

   3.  No other assignment which prefix is included in the same
       Delegated Prefix, and with a higher precedence, is being
       advertised on the same link.

6.4.  Testing Assignment's availability

   A prefix is said to be available if it does not overlap with any
   other assignment by any other router in the network.

6.5.  Accepting an Assigned Prefix

   An AP is said to be accepted when the AP is currently being
   advertised by a different router on a directly connected link, and
   will be used by the accepting router as a new Chosen Prefix.  When a
   router accepts a neighbor's assignment, it starts a timer as
   specified in Section 8.  A new CP is created from the AP, with:

   o  The same prefix.

   o  The same link ID.

   o  The authoritative bit set to false.




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   o  The same priority.

   o  The advertised bit value set as specified by the algorithm.

   o  The applied bit is unset.  It is set when the timer elapsed if the
      entry still exists.

6.6.  Making a New Assignment

   In situations where a router can make an assignment (see
   Section 6.9), the following rules are used in the following order:

   1.  If the configuration specifies a custom behavior (e.g. always
       ignore/accept a particular delegated prefix), use the
       configuration entry.

   2.  If the Delegated Prefix Preferred Lifetime is strictly greater
       than zero, an assignment MUST be made.

   3.  If no other prefix has a non-zero Preferred Lifetime, and no
       assignment is made on the link, an assignment SHOULD be made.

   4.  Otherwise, a new assignment SHOULD NOT be made.

   When the algorithm decides to make a new assignment, it first needs
   to specify the desired size of the assigned prefix.  Although this
   algorithm intends to remain generic, the use of 64 bits long prefixes
   is RECOMMENDED (See [I-D.ietf-6man-why64]).  The following table MAY
   be used as default values, where X is the length of the delegated
   prefix.

   If X <= 64:  Prefix length = 64

   If X >= 64 and X < 104:  Prefix length = X + 16 (up to 2^16 links)

   If X >= 104 and X < 112:  Prefix length = 120 (2^8 addresses per link
         and more than 2^8 links)

   If X >= 112 and X <= 128:  Prefix length = 120 + (X - 112)/2 (Link Vs
         Addresses tradeoff)

   When the algorithm decides to make a new assignment, it SHOULD first
   checks its stable storage for an available assignment that was
   previously applied on the current interface and is part of the
   current delegated prefix.  If no available assignment can be found
   that way, the new prefix MUST be randomly selected among a subset of
   available prefixes (if possible, large enough to avoid collisions).




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   Hardware specific identifiers may be used to seed a pseudo-random
   generator.

   If no available prefix is found, the assignment fails.

   The algorithm leaves much room for implementation specific policies.
   For instance, static prefixes may be configured as specified in
   Section 10.  If implemented, the router MAY also decide to execute
   the Prefix Scarcity Avoidance mechanisms, as proposed in Appendix A.

   If an available prefix is found, a new assignment is made and a new
   Chosen Prefix entry is created.

   o  The prefix value is set to the chosen prefix.

   o  The link ID is the ID of the link on which the assignment is made.

   o  The authoritative bit is set to false.

   o  The priority is set to a value between PRIORITY_AUTO_MIN and
      PRIORITY_AUTO_MAX (Section 6.8).

   o  The advertised bit is set.

   o  The applied bit is unset.  It is set when the timer elapsed if the
      entry still exists.

   A new assignment is always marked as advertised when created and
   therefore immediately provided to the flooding protocol.

6.7.  Using Authoritative Prefix Assignments

   When some authority (Delegating router, system admin, etc...) wants
   to manually enforce some behavior, it may ask some router to make an
   Authoritative Prefix Assignment.  Such assignments have their
   Authoritative bit set, SHALL NOT be overridden, and will appear in
   other router's database as Assigned Prefixes with the Authoritative
   bit set.

   There are two kinds of Authoritative Prefix Assignments.

   o  When an authority wants to assign some particular prefix to some
      interface, an Authoritative Prefix Assignment MAY be created and
      consists in a Chosen Prefix which have its Authoritative bit set
      and which is advertised.  Just like normal assignments, it MUST
      NOT be applied before the delay specified in Section 8 elapsed.





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   o  When an authority wants to prevent some prefix from being used, an
      Authoritative Assignment MAY be advertised.  Such assignments MUST
      NOT be applied and MUST be advertised through the flooding
      protocol as assigned to either no-interface, or a fake interface
      (Depending on the flooding protocol's capabilities).

   When a delegated prefix is obtained through DHCPv6 PD with a non-
   empty excluded prefix, as specified in [RFC6603], an Authoritative
   Prefix Assignment MUST be created with the excluded prefix.

      Note: If the router doesn't understand the excluded prefix DHCPv6
      option, the delegated prefix is ignored, as specified in
      Section 4.3.

6.8.  Choosing the Assignment's Priority

   When either a new Prefix Assignment is made, or an Authoritative
   Prefix Assignment is created, the creating router needs to choose
   which priority value to use.  The assignment priority is kept by the
   designated router when it starts advertising the assignment, and is
   useful when not enough prefixes are available.

   o  PRIORITY_DEFAULT SHOULD be used as default.

   o  Other values between PRIORITY_AUTO_MIN and PRIORITY_AUTO_MAX MAY
      be dynamically chosen by the implementation.

   o  Other values between PRIORITY_AUTHORITY_MIN and
      PRIORITY_AUTHORITY_MAX MUST NOT be used if not specified by an
      authority (by static or dynamic configuration).

   o  Other values are reserved.

6.9.  Prefix Assignment Algorithm steps

   At the beginning of the algorithm, all assignments that do not have
   their Authoritative bit set are marked as 'invalid', and the router
   computes for each connected link whether it is the designated router,
   as specified in Section 4.5.

   The following steps are then executed for every combination of
   delegated prefixes and interfaces.

   o  If the current interface is external, ignore that interface.

   o  If the Delegated Prefix is strictly included in another Delegated
      Prefix, ignore that delegated prefix.




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   o  If the Delegated Prefix is equal to another Delegated Prefix,
      advertised by some router with an higher router ID than the
      considered delegated prefix, ignore that delegated prefix.

   o  Look for a valid Assigned Prefix, advertised by another router on
      the current interface and included in the current Delegated
      Prefix.

   o  Look for a Chosen Prefix associated to the current interface and
      included in the current Delegated Prefix.

   o  There are four possibilities at this stage.

      1.  If no AP is found, and no CP is found, a new assignment can be
          made if and only if the router considers itself as the
          designated router.  Whether to create an assignment or not,
          and which prefix to use, is specified in Section 6.6.

      2.  If an AP is found, and no CP is found, the AP MUST be
          accepted.  The new CP's advertised bit MUST be set if and only
          if the router considers itself as the designated router.

      3.  If no AP is found, and a CP is found, the router MUST check if
          the CP's assignment is valid.  If it is, the local assignment
          is marked as valid and advertised.  If it isn't, it is
          destroyed and the algorithm applies case 1.

      4.  If both an AP and a CP are found, the router must check if the
          prefixes are the same.  If they are different and if the CP's
          Authoritative bit is not set, the CP MUST be deleted and the
          algorithm applies case 2.  If the prefixes are the same, the
          CP must be updated with the AP's priority value, marked as
          valid, and advertised if and only if the router considers
          itself as designated on the link.

   In the end all the assignments that are marked as invalid are
   deleted.

6.10.  Downstream DHCPv6 Prefix Delegation support

   If some host or non-Homenet router asks for Delegated Prefixes, a
   router MAY assign a set of prefixes and give them to the client.
   Such assignments MUST be advertised as either not assigned on any
   link, or assigned on a stub virtual link connected to the router,
   depending on the Flooding Protocol capabilities.  By default
   assignments priorities MUST be between PRIORITY_AUTO_MIN and
   PRIORITY_AUTO_MAX, SHOULD be lower than PRIORITY_DEFAULT, and the
   authoritative bit MUST not be set.  Whenever such an assignment



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   becomes invalid, DHCPv6 Reconfigure SHOULD be used in order to remove
   the prefix from DHCPv6 DP client's lease.  If DHCPv6 Reconfigure is
   not supported, leases lifetimes SHOULD be significantly small.

   Provided DPs' valid and preferred lifetimes MUST be lower or equal to
   their associated Delegated Prefix's lifetimes, and associated DHCPv6
   data SHOULD be provided to the DHCPv6 PD client.

   By default, an assigned prefix SHOULD NOT be provided to a DHCPv6 PD
   client before the apply timeout has elapsed.  But in order to allow
   faster response delay, a lease MAY first be provided with a lifetime
   of 2*FLOODING_DELAY seconds, even if the private assignments' apply
   timeout has not elapsed yet.

7.  Address Assignment Algorithm

   IPv6 routers always get at least one link-local address per link.
   Routing protocols and link DHCP servers are able to run with these
   addresses.  In some cases though, a router may need to take one or
   multiple addresses among one or multiple available Delegated
   Prefixes.  For example:

   o  The router needs connectivity to the internet (For management, NTP
      synchronization, etc...).

   o  The router needs connectivity within the home network (For
      management, DNS communications, etc...).

   o  IPv4 addresses are needed (DHCPv4, v4 link-local connectivity,
      etc...).

   When possible, SLAAC MUST be used.  In other cases a different
   mechanism is necessary for routers to get addresses.  This document
   proposes an Address Assignment Algorithm that extends the Prefix
   Assignment Algorithm and works as follows.  Each prefix assignment is
   associated with a fixed address pool, reserved for router's addresses
   assignment.  The address pool is a prefix which value is
   deterministically function of the assigned prefix.  A router MAY, at
   any time, decide to assign itself an address from any of its Chosen
   Prefixes.  Just like prefix assignments, address assignments are
   advertised to other routers and collisions are detected.  Routers
   MUST keep track of Address Assignments made by other routers on
   connected links by using information provided by the flooding
   algorithm, as defined in Section 5.5.







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7.1.  Router's address pools

   Given an assigned prefix A/X (where all A's latest '128 - X'th bits
   are set to 0), the routers reserved address pool is defined as
   follows:

   If X <= 64:  SLAAC MUST be used

   If X > 64 and X <= 110:  The pool is A/112 (2^16 addresses)

   If X >= 110 and X <= 126:  The pool is A/(X + 2) (One quarter of the
         available addresses)

   If X >= 126:  Only the designated router MAY use A/128.  Other
         routers MUST NOT get an address.

   In the case of IPv4 prefixes, the network address (first address of
   the address pool) MUST not be used.

7.2.  Address Assignment Algorithm

   In this section, we say an address assignment is made by some router
   when it intends to use, or is using the address specified by this
   assignment.  An assignment, made by some router, MUST be advertised
   on the link on which the assignment is made.  Similarly, an address
   assignment is said to be applied when the address is pushed to the
   router's interface configuration.  It is unapplied otherwise.

   Routers MUST store applied address assignments in their stable
   storage and reuse the same addresses whenever possible.  At least the
   five previously applied addresses SHOULD be stored for each
   interface.

   For a given prefix assignment, an address is said to be available if
   it is within the router's address pool associated to the prefix
   assignment, and it is not being advertised by any other router.  If
   the flooding protocol provides interface identifier in the address
   assignments, looking for collisions on considered link is enough.

   A new address assignment MUST be chosen randomly among available
   addresses.  An address assignment MUST NOT be applied when one of the
   following condition is true.

   o  The associated Chosen Prefix is not applied.

   o  The timer specified in Section 8 has not elapsed yet.





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   An address assignment must be deleted whenever one of the following
   condition becomes true.

   o  The associated Chosen Prefix is deleted or moved to another link.

   o  Some other router with a higher router ID is advertising the same
      address on the same link.

8.  Hysteresis Principle

   The IPv6 Stateless Address Autoconfiguration [RFC4862] states that
   host addresses can be kept up to 2 hours after a Router Advertisement
   wit zero lifetime is received.  Therefore, routers must be carefull
   before assigning or deprecating a prefix.

8.1.  Prefix and Address assignments

   When the flooding protocol is started, the router MUST wait
   FLOODING_DELAY before executing the prefix assignment algorithm for
   the first time.

   Prefix and address assignment algorithms are distributed.  Collisions
   may occur, but network configuration, routing protocols or upper
   layers should not suffer from these collisions.  For this reason, all
   assignments that could imply collisions are not immediately applied.

   o  A router MUST NOT apply a Chosen Prefix before it has waited
      2*FLOODING_DELAY.  If the entry is valid during the whole waiting
      time, it MUST be applied to the link it is assigned.

   o  A router MUST NOT apply an Assigned Address before it has waited
      2*FLOODING_DELAY_LL.  If the assignment is valid during the whole
      waiting time, it MUST be applied to the interface it is assigned.

8.2.  Delegated Prefixes

   Some links may be unreliable, causing repetitive connectivity loss.
   Such links shouldn't cause IP reconfiguration.

8.2.1.  Unreliable uplink

   When a router detects uplink connectivity loss, Delegated Prefixes'
   lifetimes from prefixes obtained through the uplink MUST be modified
   in the following way.

   o  The Preferred Lifetime is set to 0.





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   o  The Valid Lifetime is set to the minimum between the current Valid
      Lifetime and two hours.

   o  The default route associated with the prefix is not advertised
      anymore.

   This behavior is similar to [RFC7084] specifications and provides
   stable host configuration in case of unreliable uplink.

8.2.2.  Unreliable in-home link

   When a router stops advertising a Delegated Prefix, it MUST first
   deprecate that Delegated Prefix by advertising it for
   DP_DEPRECATE_FACTOR*FLOODING_DELAY seconds with zero valid and
   preferred lifetimes.

   When a router receives a deprecated Delegated Prefix advertisement
   from the Flooding Protocol, it MUST remove the Delegated Prefix from
   its Delegated Prefixes list.

   When a router stops receiving a Delegated Prefix from the Flooding
   Protocol, it SHOULD keep using that delegating prefix up to a period
   of min(remaining Valid Lifetime, DP_KEEP_ALIVE_TIME) seconds.

9.  ULA and IPv4 Prefixes Generation

   Although DHCPv6 PD and static configuration are regular means of
   obtaining IPv6 prefixes, routers MAY, in some cases, autonomously
   decide to generate a delegated prefix.  In this section are specified
   when and how IPv6 ULA prefixes and IPv4 private prefixes may be
   autonomously generated.

9.1.  ULA Prefix Generation

   ULA prefixes can be randomly generated as specified in [RFC4193],
   enabling stable in-home IPv6 connectivity.

   In this section, we say a ULA delegated prefix is 'stable' if it has
   been the only advertised ULA delegated prefix for at least
   2*FLOODING_DELAY seconds.  The behaviour specified in the following
   sections tend to reuse a stable ULA prefix as long as its preferred
   lifetime is not null.

   Additionaly, we say a router is the owner of a spontaneously
   generated ULA prefix if it randomly created the prefix in the first
   place.  A router SHOULD NOT create more than one prefix this way, and
   MUST remember all the prefixes they own.  As stated in the following
   sections, only the owner of a prefix can extend its lifetimes.



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9.1.1.  Choosing the ULA prefix

   When a stable ULA prefix is advertised, all routers SHOULD remember
   that prefix alongwith its associated valid and preferred lifetime.
   If this prefix stops being advertised (e.g. due to a network split)
   while its preferred lifetime is not null, the same ULA prefix SHOULD
   be selected using the same valid and preferred lifetimes.

   If there was no stable ULA prefix advertised, or if the preferred
   lifetime of the prefix was null, a prefix generated as specified in
   [RFC4193] SHOULD be used.  In case the stable storage can't be used
   or the current date cannot be determined, the prefix MAY be pseudo-
   randomly generated based on hardware specific values.

9.1.2.  Advertising a ULA prefix

   A router MAY start advertising a ULA prefix whenever the two
   following conditions are met:

   o  It is the network leader.

   o  There is no other advertised ULA prefix.

   If no IPv6 prefix is available at all, the network leader SHOULD
   start advertising a ULA delegated prefix.

   Additionaly, a router SHOULD start advertising its own ULA prefix
   whenever the three following conditions are met:

   o  A stable ULA prefix is advertised by another router.

   o  The router owns the advertised stable ULA prefix.

   o  The preferred lifetime of the advertised ULA prefix is below 10
      minutes.

   This allows a router to restart advertising a owned prefix whenever
   the preferred lifetime is approaching zero.  Which later allows him
   to extend the lifetime of the prefix.

   A router MUST stop advertising a spontaenously generated ULA prefix
   whenever one of the two following condition is met:

   o  A different ULA prefix is being advertised.

   o  The same prefix is advertised by another router, and the router
      doesn't own that prefix.




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9.1.3.  Extending prefix lifetime

   Routers MUST regularly extend the valid and preferred lifetimes of
   the ULA delegated prefix they advertise and own, so that they never
   drop to zero.

   When a router advertises a prefix it doesn't own, lifetimes are never
   extended.  When the preferred lifetime of the prefix approaches zero,
   either the owner of the prefix will start advertising the prefix with
   a non-zero preferred lifetime, or a new prefix will be generated.

9.1.4.  Authoritative ULAs

   This section doesn't prevent multiple ULA prefixes from existing
   simultaneously.  ULA prefixes may be provided by different means, as
   specified in Section 4.3.  Delegated prefixes that are delegated by a
   service provider or provisioned by an authority differ from
   'spontaneously' generated prefixes.  They MUST NOT be withdrawn if
   another ULA delegated prefix is observed.

   When at least one of such ULA prefixes is used, spontaneously
   generated ULA prefixes are withdrawned.

9.2.  IPv4 Private Prefix Generation

   A router MAY generate an IPv4 prefix when the two following
   conditions are met.

   o  It has an IPv4 address with global connectivity.

   o  No other IPv4 delegated prefix is advertised by any other router.

   A router MUST stop advertising an IPv4 prefix whenever another router
   with an higher router ID is advertising an IPv4 Delegated Prefix.

   The IPv4 private prefix must be included in one of the private
   prefixes defined in [RFC1918].  The prefix 10/8 SHOULD be used by
   default but it SHOULD be configurable.  In the case the address
   provided by the ISP is already a private address, a different private
   prefix SHOULD be used.  For instance, if the ISP is giving the
   address 10.1.2.3, 10/8 or any sub-prefix included in 10/8 SHOULD NOT
   be used.  (For instance, 172.16/12 or 192.168/16 can be selected).

10.  Manageability Considerations

   The algorithm leaves much room for implementation specific features.
   For instance, ULA prefix as well IPv4 prefix generation may be




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   disabled whenever a global IPv6 is made available.  This section
   details a few other possible configuration options.

   The implementation MAY allow each internal interface to be configured
   with a custom priority value.  The specified priority SHOULD then be
   used when creating new assignments on the given interface.  If not
   specified, the default priority SHOULD be used.

   The implementation SHOULD allow manual assignments on given links.
   When specified, and whenever such an assignment is valid, it MUST be
   advertised as Authoritative Assignments on the given interface.

11.  Documents Constants

   PRIORITY_MIN              0
   PRIORITY_AUTHORITY_MIN    4
   PRIORITY_AUTO_MIN         6
   PRIORITY_DEFAULT          8
   PRIORITY_AUTO_MAX         10
   PRIORITY_AUTHORITY_MAX    12
   PRIORITY_MAX              15

   DP_DEPRECATE_FACTOR       3
   DP_KEEP_ALIVE_TIME        60 seconds

12.  Security Considerations

   Prefix assignment algorithm security entirely relies on flooding
   protocol security features.  The flooding protocol SHOULD therefore
   check for the authenticity of advertised information.  Security modes
   may be classified in three categories.

   1.  The flooding protocol is not protected.

   2.  The flooding protocol's protection is binary: An allowed router
       may send any type of packets in the name of other routers.

   3.  All advertised messages are individually signed by the sender.

   Whenever a malicious router attacks an unprotected network, or
   whenever a malicious router is able to authenticate itself to a
   network as stated in the second case, it may for example:

   o  Prevent other routers to get a stable router ID.

   o  Prevent other routers from making assignments by claiming the
      whole available address space.




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   o  Redirect traffic to some router on the network.

   If a malicious router is able to authenticate itself in a network
   protected as in the third case, most of the previously listed attacks
   may still be performed, but traffic could only be redirected toward
   the origination of the attack, and the source of the attack could be
   identified.

   In any case, in order to protect the network, the routing protocol as
   well as the way hosts are configured also needs to be protected,
   hence requiring other link (e.g. WPA) or IP layer (e.g. IPSec-Auth
   [RFC4302] or SeND [RFC3971]) security solutions.

13.  References

13.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

   [RFC3203]  T'Joens, Y., Hublet, C., and P. De Schrijver, "DHCP
              reconfigure extension", RFC 3203, December 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.




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   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, November 2010.

   [RFC6603]  Korhonen, J., Savolainen, T., Krishnan, S., and O. Troan,
              "Prefix Exclude Option for DHCPv6-based Prefix
              Delegation", RFC 6603, May 2012.

13.2.  Informative References

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, December
              2005.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
              2008.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification", RFC
              5969, August 2010.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)", RFC
              6241, June 2011.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              November 2013.

   [I-D.ietf-homenet-arch]
              Chown, T., Arkko, J., Brandt, A., Troan, O., and J. Weil,
              "IPv6 Home Networking Architecture Principles", draft-
              ietf-homenet-arch-11 (work in progress), October 2013.



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   [I-D.ietf-homenet-hncp]
              Stenberg, M. and S. Barth, "Home Networking Control
              Protocol", draft-ietf-homenet-hncp-00 (work in progress),
              April 2014.

   [I-D.ietf-ospf-ospfv3-autoconfig]
              Lindem, A. and J. Arkko, "OSPFv3 Auto-Configuration",
              draft-ietf-ospf-ospfv3-autoconfig-06 (work in progress),
              February 2014.

   [I-D.ietf-6man-why64]
              Carpenter, B., Chown, T., Gont, F., Jiang, S., Petrescu,
              A., and A. Yourtchenko, "Analysis of the 64-bit Boundary
              in IPv6 Addressing", draft-ietf-6man-why64-06 (work in
              progress), October 2014.

   [I-D.arkko-homenet-prefix-assignment]
              Arkko, J., Lindem, A., and B. Paterson, "Prefix Assignment
              in a Home Network", draft-arkko-homenet-prefix-
              assignment-04 (work in progress), May 2013.

   [I-D.bhandari-dhc-class-based-prefix]
              Systems, C., Halwasia, G., Gundavelli, S., Deng, H.,
              Thiebaut, L., Korhonen, J., and I. Farrer, "DHCPv6 class
              based prefix", draft-bhandari-dhc-class-based-prefix-05
              (work in progress), July 2013.

   [I-D.chelius-router-autoconf]
              Chelius, G., Fleury, E., and L. Toutain, "Using OSPFv3 for
              IPv6 router autoconfiguration", draft-chelius-router-
              autoconf-00 (work in progress), June 2002.

   [I-D.dimitri-zospf]
              Dimitrelis, A. and A. Williams, "Autoconfiguration of
              routers using a link state routing protocol", draft-
              dimitri-zospf-00 (work in progress), October 2002.

Appendix A.  Scarcity Avoidance Mechanism

   Although an ISP should provide enough addresses, an implementation
   must carefully manage the provided address space.  First, when a new
   assignment is made, the prefix should be selected amongst a set of
   prefixes so that prefix waste is minimized.  Then, a router may
   decide to execute procedures intended to avoid prefix scarcity.
   Different approaches are possible.  This section intends to provide
   guidelines for such procedures.  They are optional and are compatible
   with routers that only support basic requirements defined in this
   document.



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A.1.  Prefix Wasts Avoidance

   Given a Delegated Prefix, different routers may try to assign
   prefixes of different lengths.  Particularly, a non-homenet
   downstream router may ask for a delegated prefix of significant size,
   as specified in Section 8.2.  Some other routers, like sensors, may
   also require small prefixes.  When randomly selected, a few /80s may
   easily prevent the assignment of bigger prefixes.  Small prefixes
   should therefore be selected in neighboring areas.

   For instance, given a delegated prefix 2001::/56 and an assigned
   prefix 2001::/64, the best prefix choice in order to reduce prefix
   space waste is 2001:0:0:1::/64.  Other choices are then to be taken
   in 2001:0:0:2::/63, 2001:0:0:4::/62, 2001:0:0:8::/61, etc...

   Creating an efficient prefix selection algorithm may be challenging
   as it needs to fullfill somehow contradictory requirements:

   1.  The prefix MUST be chosen amongst available prefixes, which
       implies that other routers may interfere with the process.

   2.  The prefix MUST be chosen randomly in a subset of available
       prefixes.  When possible, the subset must be big enough to avoid
       collisions.

   3.  The prefix SHOULD be selected amongst prefixes that reduces the
       prefix space waste.

   4.  The prefix SHOULD be selected pseudo-randomly.

   The following algorithm offers a satisfying tradeoff.  Given a
   Delegated Prefix and the desired prefix length:

   1.  Compute the minimal subset of available prefixes included in the
       Delegated Prefix.  In the example given previously, the minimal
       subset was {2001:0:0:1::/64, 2001:0:0:2::/63, ..., 2001:0:0:80::/
       57}.

   2.  Compute the set of prefixes of desired length so that:

       *  It contains exactly RANDOM_SUBSET_SIZE prefixes, or all the
          available prefixes if there are less than RANDOM_SUBSET_SIZE
          available prefixes.

       *  Prefixes are picked in the prefixes from the minimal subset of
          available prefixes which lengths are the highest.





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       *  When multiple subsets are possible, privelege lexicographicaly
          lowest prefixes.

       If RANDOM_SUBSET_SIZE equals 10, the subset would be
       {2001:0:0:1::/64, 2 /64s in 2001:0:0:2::/63, 4 /64s in
       2001:0:0:4::/62, the 3 first /64s in 2001:0:0:8::/61}.

   3.  First try PSEUDO_RANDOM_TENTATIVE pseudo-random prefixes,
       computed from the DP, with the given length, based on interface
       specific hardware values (For instance using values generated
       like HASH(MAC Address : Counter).  The hash function doesn't need
       to be cryptographic).  The first prefix amongst this set that
       also is in the set computed at step 2 is chosen.  If no prefix is
       found, try next step.

   4.  Choose a prefix randomly among prefixes in the subset computed at
       step 2.

   This algorithm, defined as a sequence of prefix sets computation, may
   seem algorithmicaly complex, but it can be efficiently implemented.
   The key element in order to do so is the ability to iterate
   efficiently over all the available prefixes.

   RANDOM_SUBSET_SIZE should provide sufficiently low collision
   probability.  A value of 256 should be enough in most cases.
   PSEUDO_RANDOM_TENTATIVE is purely implementation dependent, but
   shouldn't be too high as the probability of finding an available
   prefix that way quickly decreases with the number of used prefixes.
   A value of 10 should be sufficient.

A.2.  Increasing Assigned Prefix Length

   When a new assignment can't be created, and if not forbidden by the
   router's configuration, the router MAY increase the size of the
   desired prefix.  For instance, if an available /64 can't be found,
   the router may look for a /80.  Nevertheless, this implies using
   DHCPv6 instead of SLAAC, which SHOULD be avoided.

A.3.  Foreseeing Prefixes Exaustion

   The previously proposed solution may be useful in some particular
   cases, but won't work when no more prefixes are available.  A router
   MAY try to detect when default length prefixes are becoming rare.  In
   such a situation, it MAY decide to allocate a longer prefix, part of
   an available shorter prefix.  For instance, if A/64 is available, but
   there are not many other available /64, the router can try to
   allocate A/80.  If the allocation doesn't raise any collision, this




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   procedure will prevent A/64 from being used by other hosts, hence
   creating a large set of smaller available prefixes to be used.

   Such an allocation is considered dynamic.  The Authoritative bit MUST
   NOT be set and the priority MUST be among values authorized as
   dynamically chosen in Section 6.8.

   When different prefixes lengths are being used, the random prefix
   selection MUST NOT be uniform among all possibilities.  Instead, it
   SHOULD privilege prefixes contained in bigger prefixes that cannot be
   allocated.  For instance, if 2001::/56 is the DP, and 2001:0:0:0:1::/
   80 is an assigned prefix, other /80 should be randomly chosen in
   2001:0:0:0:1::/64 before being chosen in other /64s.

A.4.  Cutting an Existing Assignment

   When specifically required by an authority (configuration or DHCP), a
   router MAY decide to un-assign one of its own assignment, in order to
   cut it in smaller prefixes, or to send an overriding assignment in
   order to force the network to stop using a particular prefix.
   Because such a procedure may imply links reconfiguration, it SHOULD
   be avoided whenever possible.

   Such allocation are considered as required by an authority.  The
   Authoritative bit MAY be set and the priority MUST be among values
   authorized as specified by an authority in Section 6.8.

   As an example, if a router can't find a /64 for a link that, with a
   high priority, must be given a /64, it chooses a prefix assigned by
   some other router, to another link, with a lower priority, and
   creates a new Chosen Prefix with a higher priority.  The other router
   will be forced to remove its own assignment, hence making the new
   assignment valid.

Appendix B.  Acknowledgments

   This document is the continuation of the work being done in
   [I-D.arkko-homenet-prefix-assignment].  The authors would like to
   thank all the people that participated in the previous document's
   development as well as the present one.  In particular, the authors
   would like to thank to Tim Chown, Fred Baker, Mark Townsley, Lorenzo
   Colitti, Ole Troan, Ray Bellis, Markus Stenberg, Wassim Haddad, Joel
   Halpern, Samita Chakrabarti, Michael Richardson, Anders Brandt, Erik
   Nordmark, Laurent Toutain, Ralph Droms, Acee Lindem and Steven Barth
   for interesting discussions in this problem space.  The authors would
   also like to point out some past work in this space, such as those in
   [I-D.chelius-router-autoconf] or [I-D.dimitri-zospf].




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Authors' Addresses

   Pierre Pfister
   Cisco Systems
   Paris
   France

   Email: pierre.pfister@darou.fr


   Benjamin Paterson
   Cisco Systems
   Paris
   France

   Email: benjamin@paterson.fr


   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@piuha.net



























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